1. Field of the Invention
This invention generally relates to a lamp circuit in which an AC power source is connected to a plurality of lamps through a plurality of isolation transformers, respectively, and more particularly to a lamp circuit with a disconnected lamp detecting device.
2. Description of the Prior Art
A conventional thyristor-type constant current regulator (hereinafter designated as CCR), as shown in FIG. 1, has been employed as a power supply for a lamp circuit for use in lighting an airport runway.
In FIG. 1, numeral 1 designates an AC power source. Numeral 20 designates a thyristor-type CCR, wherein 2 designates a smoothing reactor, 3 and 4 designate thyristors, 5 designates an output transformer, 6 designates a current transformer, 7 designates a differential amplifier, 8 designates a gate control circuit, and 13 designates a reference current input adjuster. Numeral 9 designates a potential transformer, 10 designates a disconnected lamp detecting device, and 11 designates an alarm device. Numeral 12 designates a series lamp circuit which comprises a plurality of series connected isolation transformers 121, the primary windings of which are connected in series with each other. The secondary winding of each transformer 121 is connected to a lamp 122. The series lamp circuit 12 is connected to the output transformer 5.
As shown in FIG. 1, the output current i of the thyristor-type CCR 20 is detected by the current transformer 6 and is compared with the signal Cs of the reference current input adjuster 13 in the differential amplifier 7. The differential amplifier 7 amplifies the compared signal and produces a signal Go, which is applied to the gate control circuit 8. The gate control circuit 8 produces gate signals G1 and G2, in response to the signal Go, which are supplied to the respective gates of the thyristors 3 and 4 so as to maintain the output current of the thyristor-type CCR 20 at a constant level, i.e., to keep the intensity of the lamps 122 at a constant level.
One example of the disconnected lamp detecting device 10 is shown in FIG. 2 in detail. After the voltage signal v of the potential transformer 9 and the current signal i of the current transformer 6 are rectified by respective full-wave rectifiers D.sub.1 and D.sub.2, the difference signal e between the two outputs of the rectifiers D.sub.1 and D.sub.2 is produced. After smoothing the difference signal e, the smoothed signal is supplied to the base terminal of a transistor Tr which produces an alarm signal A to activate the alarm device 11 when the value of the smoothed signal exceeds a predetermined value. The alarm device 11 indicates the alarm condition by means of a buzzer or a lamp in response to the alarm signal A.
In the case where no lamp 122 is disconnected, the voltage signal v and the current signal i become waveforms v.sub.1 and i.sub.1, respectively, as shown in FIGS. 3(a) and 3(b). Therefore, the difference signal e between these signals becomes the waveform e.sub.1 shown in FIG. 3(c). At this time since the transistor Tr is not turned on, the alarm signal A is not produced.
If it is assumed that a number of lamps 122 are disconnected, the voltage signal v and the current signal i become the waveforms v.sub.2 and i.sub.2, respectively, shown in FIGS. 3(d) and 3(e) wherein the waveform v.sub.2 rises sharply while waveform i.sub.2 rises more slowly. Therefore, the difference signal e becomes the waveform e.sub.2 shown in FIG. 3(f). The difference signal e is smoothed and applied to the transistor Tr which turns on thereby producing the alarm signal A.
The detection of the disconnected lamps is thus carried out. However, the waveforms of the voltage signal v and the current signal i are analog signals, and often deformed by disturbances, such as noise, which are characteristic of analog signals. Therefore, even though a lamp is not actually disconnected, the voltage value of the smoothed difference signal reaches a value sufficient to operate the transistor Tr of the disconnected lamp detecting device 10. As a result, a false alarm signal A is produced.
To prevent such an above-mentioned misdetection, the operating voltage value, which makes the transistor Tr operate, must be set to a large value; however, this makes it impossible to detect disconnecting lamps with high-sensitivity. Therefore, the sensitivity of the detection is limited to about ten percent of the rated load, and thus the desired sensitivity of detection within a limit of about five percent of the rated load cannot be achieved.
There is the danger of increasing the risks to aircraft due to a defect in the runway lighting of an airport. Moreover, when an isolation transformer, in which the secondary winding has been opened by a disconnected lamp, is left for a long period of time, there arises a danger of a winding short upon the application of a high-voltage pulse and the danger of a burn-out due to rising temperature. Furthermore, to display the number of actually disconnected lamps in addition to the alarm function, it is necessary to provide a new display circuit.
Moreover, even when the disconnected lamp is detected, it is impossible to exchange the disconnected lamp with a new one while the series lamp circuit is energized because the disconnected lamp is located on the runway and changing it is very dangerous. Also, in order to detect the disconnected lamp it is necessary that the series lamp circuit be lighted; therefore, detection of the disconnected lamp is performed only when the series lighting circuit is in an energized condition.